Full duplex transceivers as used in digital wireless communication systems comprise transmitter and receiver devices. In such transceivers, signals are simultaneously transmitted and received via an air interface. In the RF domain of full duplex transceivers, the transmitted and received RF signals are spaced apart from each other by a certain frequency, the so-called “duplex distance”.
FIG. 1 shows a schematic block diagram illustrating a conventional full duplex transceiver. The transceiver comprises a transmitter (TX) path and a receiver (RX) path. A duplexer is coupled between the TX path, the RX path and an antenna. The duplexer separates RF signals from the TX path to the antenna and RF signals received via the antenna to the RX path.
The TX path comprises a baseband signal processing stage and an RF signal processing stage. The baseband signal processing stage comprises an in-phase (I) signal branch and a quadrature-phase (Q) signal branch. Each of the I-branch and the Q-branch comprises a digital-to-analog converter (DAC), a filter and a mixer. The mixer is usually implemented as an IQ-modulator and converts low-frequency baseband signals into RF signals. The output signals from the mixer in the I- and Q-branches are provided to the RF signal processing stage.
The RF signal processing stage comprises a signal combining component, a variable gain amplifier (VGA), a surface acoustic wave (SAW) filter and a power amplifier (PA). The signal combining component combines the output signals from the I- and Q-branches into one signal and provides the signal to the VGA. The VGA amplifies the combined signal to a wanted power level and feeds it to the PA. The signal amplified by the PA is then provided to the duplexer which couples the signal to the antenna. The RX path front-end comprises a low noise amplifier (LNA) which is supplied via the duplexer with signals received by the antenna.
In the full duplex transceiver shown in FIG. 1, noise generated within the TX path, i.e. noise generated by the on-chip integrated blocks, leaks through the duplexer into the RX path, as indicated by the semi-circular arrow. In order to reduce the noise leakage from the TX path via the duplexer into the RX path, the SAW filter is placed in the TX path between the VGA and the PA.
However, SAW filters are rather expensive. Furthermore, filters generally require space on a circuit platform, whereas it is a general design aim to reduce the size of transmitter devices, e.g. for usage in mobile applications. Moreover, for multi-band transmitter devices, one filter has to be provided for each band. Thus, the costs and the space required for external filters increases with the number of frequency bands.
An alternative way to decrease the noise leakage from the TX path through the duplexer into the RX path is to increase the power consumption in the TX path. Thereby, the noise generated in the TX path is generally reduced. However, increasing power consumption is not desirable for transmitter devices, in particular battery-powered devices like handheld devices.
The article “A low-power WCDMA transmitter with an integrated notch filter” by Ahmad Mirzaei and Hooman Darabi in “Proceedings of the 2008 IEEE International Solid-State Circuits Conference”, pages 212 to 213, discloses a configuration of a Wideband Code Division Multiple Access (WCDMA) transceiver device which is providing noise cancellation without relying on SAW filters. FIG. 2 shows a schematic block diagram illustrating this WCDMA transceiver device configured to reduce transmitter-to-receiver noise leakage.
The transceiver shown in FIG. 2 deviates from the transceiver shown in FIG. 1 in that no SAW filter is included in the TX path between the VGA and the PA. Instead of the SAW filter, a feedback loop is provided around the VGA. Within the feedback loop, noise is removed. In particular, the feedback loop comprises an in-phase branch for I-signals and a quadrature-phase branch for Q-signals. Each of the in-phase branch and the quadrature-phase branch comprises a first mixer MX1 for downconverting the I- and Q-signals based on a receiver local oscillator signal RX LO from RF frequency to baseband frequency, a low-pass filter LPF for filtering the wanted TX signal from the downconverted signal and a second mixer MX2 for upconverting the filtered signal from baseband frequency back to RF frequency.
As can be seen from the first frequency diagram depicted in the feedback loop of FIG. 2, the LPFs produce sharp band-pass filters centred at the RX frequency fRX. The LPFs selectively filter noise generated in the RX frequency band. In order to avoid noise leakage into the RX frequency band, the LPFs have a bandwidth which is larger than the RX baseband bandwidth. After the upconversion by the second mixer MX2, the noise power is fed back to the VGA. As can be further seen from the second frequency diagram shown in the upper region of FIG. 2, the VGA is provided with an output impedance which is zero at the RX frequency fRX. Thus, noise in the RX frequency band can be attenuated by the frequency-selective feedback loop shown in FIG. 2.
However, transmitter-to-receiver noise leakage is most problematic for short duplex distances. For example, transceiver devices which are operating in accordance with the Third Generation Partnership Project Long Term Evolution (3GPP LTE) standard use signal band-widths of up to 10 MHz together with a short duplex distance. For short duplex distances, the LPFs shown in FIG. 2 have to be sharp high-order low-pass filters. However, high-order low-pass filters, in particular sharp high-order low-pass filters, are generally avoided in feedback systems, since they cause stability problems due to their phase change.
Document U.S. 2007/0165745 A1 concerns a radio transmitter with IQ-modulator error compensation. The transmitter comprises a homodyne observation receiver producing a first real base band signal from a real radio frequency signal, means for converting a complex baseband signal into a second real baseband signal, an adaptor for determining parameters controlling an IQ-error compensator by minimizing the error between the first and second real baseband signals, and means for analogue signal processing of the real radio frequency signal to compensate for the fact that the homodyne observation receiver produces a real and not a complex signal.
Document WO 2008/088603 concerns a wireless communication unit comprising a linearized transmitter having a forward path and a feedback path, respectively comprising at least one up-mixer and down-mixer and forming two loops in quadrature. A phase training signal is applied to the at least one down-mixer in the feedback path in an open loop mode of operation to identify a loop phase adjustment to be applied.
Document U.S. 2007/0184782 A1 concerns an apparatus for cancelling intermodulation interference at baseband of a receiver, comprising a reconstruction circuit configured to receive a first signal, approximate the linear and non-linear characteristics of a leakage path from a transmitter to the receiver, and provide a reconstructed output signal, and a signal adder configured to receive the reconstructed output signal and subtract it from a second signal from the receiver.